The present invention relates to a non-full state detecting apparatus and methods and more particularly to improvements to an apparatus and method for detecting whether a test conduit is full of a fluid flowing therethrough or not.
As shown in FIG. 1, JP-A-59-2301158 discloses a conventional electromagnetic flowmeter which is provided with excitation coils 2, 3 disposed respectively above and below a test conduit 1, a power source 6 and a changeover switch 7 which selectively drive those coils. When a ratio V.sub.U /V.sub.L is equal to, or less than, a given value where V.sub.U is an output from a pair of horizontally opposing electrodes 4, 5 disposed on the inner surface of the test conduit 1 under the condition where the excitation coil 2 disposed above the conduit 1 is driven, and V.sub.L is an output from the pair of electrodes 4, 5 under the condition where the excitation coil 3 disposed below the conduit 1 is driven, an operation control unit 11 determines that the conduit 1 is in a non-full state where the conduit is not full of a fluid and supplies a predetermined output, for example, to an alarm (not shown) to drive same and alert the user to that state.
Each time the changeover switch 7 is switched with a timing pulse generated by a pulse generator 8, the upper and lower excitation coils 2 and 3 are selectively driven. Even when any one of the coils is driven, an electromotive force is generated across the opposing electrodes 4 and 5.
The electromotive force, or the sensor output, is amplified by an amplifier 9, the output of which is converted by an A/D converter 10 to a digital one, which is then delivered to the operation control unit 11 which includes a CPU. The operation control unit 11 calculates a flow in a pre-stored program and sends data on the calculated flow, for example, to a controller, (not shown) which needs the data, through a D/A converter (not shown), if necessary.
The operation control unit 11 also provides control over the pulse generator 8 and invariably monitors the outputs obtained across the electrodes 4 and 5 when the respective upper and lower excitation coils 2 and 3 are selected and driven.
Generally, the generated electromotive force from the electromagnetic flowmeter sensor is not proportional to the average flow velocity when the flow velocity distribution is vertically asymmetrical.
Thus, when in FIG. 1 a non-full fluid 12 flows through the conduit 1 with an air layer 13 being produced above the fluid within the conduit 1, the flow velocity distribution becomes asymmetrical vertically with a lower larger distribution.
The magnetic field strength distribution obtained when the upper excitation coil 2 is driven is not the same as that obtained when the lower coil 3 is driven. The distribution is heavy in its upper portion when the upper coil 2 is exited and the distribution is heavy in this lower coil 3 is exited (see Fig.2).
If conduit 1 is in a full state where the conduit is full of a fluid, both the sensor outputs are the same in any of these two cases. If the conduit is in a non-full state, the degrees of contribution of the magnetic strength of the upper and lower coils to the output are not equal and the ratio of the output V.sub.U obtained when the upper coil is driven to the output V.sub.L obtained when the lower coil is driven is less than 1 (V.sub.U /V.sub.L &lt;1).
Thus, the outputs obtained when both the coils are driven are invariably monitored, and the user is alerted to that fact when the ratio of those outputs becomes a given value or less.
The electromagnetic flowmeter normally operates in a state full, so that when such an alarm is generated, or when the conduit is in a non-full state, the operator takes proper steps such as stoppage of the flow measurement.
The principles of the electromagnetic flowmeter are as follows. The relationship between the flow and the output of a sensor obtained when the upper coil for the test conduit is excited is beforehand obtained. The flow is calculated from the obtained sensor output on the basis of the above relationship. (Concerning the details of such principles, see U.S. Ser. No. 07/968508, filed on Oct. 29, 1992 whose assignee is the same as in this application). This earlier application also discloses an electromagnetic flowmeter which operates also in a non-full state. This earlier application is incorporated herein by reference.
In the non-full state flowmeter of FIG. 1, the magnetic flux distribution in the conduit produced when each of the upper and lower coils 2 and 3 is excited is vertically asymmetrical, as shown in FIG. 2, so that detection may involve errors.
The above conventional technique has the problem that responsiveness and the stability of the zero point are low, which will be described below.
(a) Low responsiveness
When the upper and lower excitation coils 2 and 3 are alternately driven at periods of excitation t and the flow is to be calculated on the basis of outputs V.sub.U and V.sub.L deriving when the corresponding coils are excited, as shown in FIG. 3, a time 2t which is twice the period of excitation t as shown in FIG. 3 is required for the measurement of the flow. This is equivalent to a change of the period of excitation to 2t, which implies a deterioration in the responsiveness to a change in the flow.
(b) Low stability of the zero-point
In order to solve the problem of the responsiveness in the (a), the period of excitation t is required to be reduced to t/2. It is well known that the stability of the zero point of the electromagnetic flowmeter is deteriorated as the period of excitation is reduced.
This is because the cause of unstableness of the zero point derives from simultaneous sampling of the flow signal and 90.degree.-noise which is superposed on the flow signal and attenuated exponentially. If the period of excitation is long, the flow signal can be sampled when the 90.degree.-noise is low, as shown in FIG. 4(II), while if the period of excitation is shorter, a relatively large 90.degree.-noise portion is inevitably sampled, as shown in FIG. 4(I).
As just described above, the conventional technique cannot avoid either one of the above problems (a) and (b). SUMMARY OF THE INVENTION
It is an object of the present invention to solve at least one of the above problems.
It is another object of the present invention to provide a non-full state detecting apparatus and method and apparatus having a new structure.
It is a further object of the present invention to provide a non-full state detecting apparatus and method involving a reduced small detection error.
It is a still further object of the present invention to provide a non-full state detecting apparatus and method which provides high responsiveness.
In order to achieve at least one of the above objects, according to the present invention, the output (first output) of a sensor obtained when both an upper and a lower coil are excited and the output (second output) of the sensor obtained when either the upper or lower coil is excited are compared.
The detecting apparatus according to the present invention may share its components with the electromagnetic flowmeter. Thus, in the measurement of the flow, both the upper and lower coils are excited and the flow of the fluid is estimated from a sensor output in accordance with the beforehand prepared relationship between output and flow. The excitation of both, the upper and lower coils, brings about a vertically and horizontally symmetrical distribution of magnetic field strength in the conduit. When a non-full state detection is desired to be performed in the measurement of the flow, the ratio of an output from the opposing electrodes, or a sensor, on the basis of the magnetic field from one of the coils under the condition where excitation of the other of the coils is stopped to an output from the opposing electrodes when both coils are excited is calculated. When the ratio changes by more than a constant value compared to that deriving when the conduit is full of the fluid, it is determined that the conduit is not full of the fluid.
Alternatively, when a non-full state detection is desired to be performed, the ratio of an output from the opposing electrodes on the basis of the magnetic field of the upper coil under the condition where excitation of the lower coil is stopped to an output from the opposing electrodes under the condition where both the upper and lower coils are excited is calculated. When the ratio is lower than a constant value, it may be determined that the conduit is not full of the fluid.
Alternatively, when a non-full state detection is desired to be performed, the ratio of an output from the opposing electrodes on the basis of the magnetic field of the lower coil under condition where excitation of the upper coil is stopped to an output from the opposing electrodes under the condition where both the upper and lower coils are excited is calculated. When the ratio exceeds a constant value, it may be determined that the conduit is not full of the fluid.
Since in any case both the upper and lower coils are excited, the interval in which the magnetic strength in the conduit is asymmetrical is reduced. Thus, the error involved is reduced compared to the conventional non-full state detecting apparatus.
The non-full state can be detected by simply stopping the excitation of one of the upper and lower coils excited in the normal operational state. Therefore, responsiveness is improved. Thus, the excitation period can be increased and hence the stability of the zero point is ensured.